1. Field of the Invention
The present invention generally relates to a radio access system, and more particularly, to a radio access system for a telematique service which is constructed with subscriber lines from an exchanger to subscribers' premises through radio lines.
2. Description of the Related Art
Recently, for constituting the subscriber line from the exchanger to each subscriber's premise, instead of a wire line such as a copper cable or an optical fiber cable, a radio line system is being used. This system is referred to as a wireless local loop (WLL) system. In the WLL system, an air interface used in mobile communication systems (TDMA system which is used in a personal digital cellular system (PDC) and a personal handy phone system (PHS) in Japan, CDMA system, etc.) is used for a radio line part. Further, for a subscriber terminal, a conventional telephone unit is commonly used through a mobile communication subscriber terminal or a dedicated WLL terminal.
The WLL system is provided for replacing the wire-line system as an access system which accommodates subscribers with rapid service at a low cost. Therefore, for services provided in the WLL system, substantially the same convenience as that of the conventional wire telephone is expected. For example, in the WLL system, telecommunication services except a telephone service (which are referred to as "telematique services", such as a facsimile service and a personal-computer communication service, the term "telematique" was made in CCITT) need to be carried out with the same protocol and apparatus as those of the conventional telephone.
The WLL system is constructed based on the mobile communication system. However, the WLL system is not a perfect duplicates of the mobile communication system, but has the following differing features.
As the first feature, though the mobile communication system is constructed in a dedicated mobile communication network which is independent from a public switched telephone network (PSTN), the WLL system is constructed as a part of the PSTN. Therefore, the mobile communication system exclusively uses mobile exchangers in the mobile communication network, but the WLL system is connected to a conventional subscriber exchanger in the PSTN.
As the second feature, the mobile communication system supports only dedicated mobile communication subscriber terminals, but the WLL system supports the conventional wire telephone used in the PSTN as well as the mobile communication subscriber terminal.
These two features may be a large disadvantage when the telematique service dealing with a data signal from such as a facsimile machine or a MODEM is carried out in the WLL system. FIG. 1 shows a basic configuration of the prior-art WLL system.
In FIG. 1, a subscriber terminal SU 10 is provided at the subscriber's premise, and has a modular jack so as to be usable as a conventional telephone. The air interface is provided between the SU 10 and a base terminal station BTS 12. In a speech coding-and-decoding device (CODEC) inside the SU 10, highly efficient coding and decoding of speech are carried out. A telephone TEL 11 is a conventional wire telephone. The BTS 12 accommodates a plurality of SUs 10, and converts the air interface and a base station controller (BSC) interface to each other. The BSC 13 accommodates a plurality of BTSs 12, and has a converting function of a WLL-side interface and a PSTN-side interface. The BSC 13 further has connecting/handing-off/call-processing functions and a monitoring function.
A PSTN 14 is a conventional network, and accommodates the PSTN subscriber telephones TEL 11 and the WLL system through subscriber local exchangers (LEs). The LE 15 is an exchanger for connecting the WLL system to the PSTN, and commonly accommodates the PSTN subscriber telephones.
In the above-discussed configuration of the WLL system, only a speech service is supported. Namely, since the CODEC is provided in the SU 10 and the BSC 13, the telematique service for such as a facsimile machine cannot be provided.
In the following, before considering the telematique service in the WLL system, how the telematique service in the mobile communication system is carried out will be studied.
In the mobile communication system, the dedicated mobile communication network is provided independent from the PSTN, and is connected to the PSTN through a mobile gate switching system.
FIG. 2 shows a illustration for explaining the telematique service in the mobile communication system. In FIG. 2, a portable set PS 20 is connected to a base terminal station BTS 12a through an air interface, and has functions of compressing a subscriber speech and supplying the compressed signal to a traffic channel of the radio lines.
A telematique-service adaptor ADP 21 is connected between the PS 20 and one of a FAX 22 and a MODEM (modulator and demodulator) when the telematique data communication using facsimile transmission and reception or the MODEM is carried out. The ADP 21 carries out a converting operation of the MODEM protocol and a disconnecting control of the CODEC in the PS 20. Therefore, two kinds of ADPs for the facsimile and the MODEM are provided.
The BTS 12a establishes radio links with a plurality of PSs 20, and is connected to a base station controller BSC 23. The BSC 23 accommodates a plurality of BTSs 12a, and has connecting/handing-off/call-processing functions and a monitoring function.
A mobile switching center MSC 24 (which corresponds to the mobile switching system) carries out line connecting and switching operations, and includes an inter-working function IWF 25. The IWF 25 is a FAX/PC adaptor, and controls conversion of the modem protocol and disconnection of the CODEC. The IWF 25 is operable for both protocols of the facsimile machine and the MODEM.
A mobile gate switching system MGS 26 is located between the mobile communication network and one of another mobile communication network and a PSTN 27, and carries out an interactive line switching operation. The PSTN 27 accommodates a PSTN subscriber telephone 29 through an LE 28. Further, the PSTN 27 is connected to the mobile communication network through the MGS 26.
As discussed above, in the mobile communication system, the IWF 25 is additionally provided in the MSC 24, and by the subscriber connecting the FAX 22 to the PS 20 through the ADP 21, a transparent line for the FAX 22 is established between the IWF 25 and the ADP 21. In this way, the facsimile transmitting-and-receiving operation may be carried out.
In the following, a description will be given of how the telematique service in the WLL system is carried out.
FIG. 3 shows an illustration for explaining a prior-art method of carrying out the telematique service in the WLL system. As shown in FIG. 3, in the SU 10, in addition to the modular jack for connecting the conventional telephone, a dedicated connector for connecting a mobile communication adaptor ADP 31 is provided. Namely, in the SU 10, the same function as that of the PS 20 may be provided. Therefore, to the SU 10, both the wire telephone and the ADP 31 may be connected. In this case, for the BSC 13, the same function as that of the IWF 25 is required.
The above-discussed configuration of the WLL system is one method of carrying out the facsimile transmitting-and-receiving operation in the WLL system. However, at present, a telephone combined with a facsimile (referred to as a facsimile-combined telephone) is in wide commercial use. When the conventional telephone TEL 11 is replaced with the facsimile-combined telephone, the facsimile-combined telephone needs to be connected to the SU 10 through the modular jack interface connection. In this case, the ADP 31 becomes unusable. Therefore, there is a problem in that in the above-discussed configuration of the WLL system, a facsimile function of the facsimile-combined telephone may not be used.
In the above-discussed configuration, when the facsimile function is operated in a speech path of the telephone, since the CODEC for coding and decoding the speech is provided in the speech path, the telematique signal such as a facsimile signal and a MODEM signal is distorted due to the CODEC.
In the same way, since a dual-tone-multi frequency (DTMF) signal produced from a push-button phone is also transmitted through the CODEC, the waveform of the DTMF signal may be distorted. Therefore, there is a problem that in the prior-art WLL system, an airline ticket reservation cannot be carried out due to the DTMF signal on an up-link line being distorted, and reproduction of a stored message in an answer phone cannot be carried out due to the DTMF on a down-link line being distorted.